In medieval and modern usage, a demon is an evil or malicious spirit. The word demon was first adapted in the New Testament from the earlier Greek word daemon. The Greek word meant something different from the medieval notion of demons, and some scholars argue that Jews and Christians in the 1st century used the word demon in its original Greek sense, rather than in the later medieval sense. In most polytheistic religions, the distinction between demons, gods and demigods can be fairly vague, and not all demons would be considered malevolent.

In contemporary Christianity, demons are generally considered to be angels who fell from grace by rebelling against God. However, this view, championed by Origen, Augustine and John Chrysostom, arose during the 6th century. Prior to that time, the primary sin of fallen angels was considered to be that of mating with mortal women, giving rise to a race of half-human giants known as the Nephilim.